One type of cellular communication system employs a technology known as CDMA (code division multiple access). In this system, users are distinguished from one another by unique codes assigned to them while sharing the same frequency band at the same time. CDMA converts information to be transmitted into a spread spectrum signal by multiplying the information by a pseudo random number (PN). This is what spreads the spectrum. The pseudo random number is a digital piece of data that, for all intents and purposes, appears to be entirely random. This PN is synchronized to the system in the CDMA mobile station by a pilot carrier sent from the base station. The rate at which the PN sequence is generated is known as the PN chip rate. For further description of CDMA and the differences between CDMA, TDMA (time division multiple access), and FDMA (frequency division multiple access), please see “Code Division Multiple Access,” Communications, Feb. 1990, by Fred Baumgartner, hereby incorporated by reference.
FIG. 1 graphically illustrates a CDMA-based communication system comprising a mobile station 10, a base station 20, reverse link 30 which represents the electromagnetic wave communication link transmitted from mobile station 10 to base station 20 and forward link 40 which represents the electromagnetic wave communication link transmitted from base station 20 to mobile station 10.
Current mobile stations used in wireless communications have symmetric forward and reverse channel bandwidths and corresponding symmetric chip rates. The term 'symmetric means that the bandwidths for the forward and reverse channels are the same and that the chip rates in the forward and reverse directions are the same. A CDMA standard, W-CDMA, has recently been proposed to accommodate high speed data (HSD) communication service over an air or wireless link. The HSD service may include various applications such as Internet access, electronic mail service, multimedia applications where more throughput is required for the forward channel than for the reverse channel, etc. Many of these services, such as multimedia applications, require asymmetric bandwidths for the forward and reverse directions. The term “asymmetric” means that the PN chip rates on the forward and reverse links are different from each other.
In present CDMA-based communication systems, all of the mobile stations have symmetric bandwidths for the forward and reverse links. The call setup procedure between a mobile station and a base station is accomplished over a symmetric forward and reverse link. After the call setup procedure is completed, a call is maintained on the same forward/reverse channel. FIG. 2 illustrates asymmetric channel allocation for the forward and reverse channels wherein F0 (formed link channel frequency) does not equal F1 (reverse link channel frequency) and BW0 (forward link channel bandwidth) is equal to BW1 (reverse link channel bandwidth). Both call setup and call maintenance are performed over the F0, F1 band.
What is needed are mobile stations that accommodate high speed data communication services by supporting asymmetric forward and reverse channel allocation.